Method of fabricating liquid crystal display device

ABSTRACT

Disclosed is a method of fabricating a liquid crystal display device enabling to form a uniform gate insulating layer in thickness. The present includes the steps of forming a gate line, a gate electrode, and a storage line on a substrate and forming a gate insulating layer on the substrate including the gate line and the gate electrode using first and second gases having a gas mixture ratio of 0.3˜0.5:1. And, the first and second gases are mono-silane(SiH 4 ) and ammonia(NH 3 ), respectively. Accordingly, the present invention enables a uniformly thick gate insulating layer, thereby to improving the discharging time as well as reducing flicker on the screen.

[0001] This application claims the benefit of the Korean Application No.P2001-86754 filed on Dec. 28, 2001, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of fabricating a liquidcrystal display device enabling formation a uniform gate silicon nitrideinsulating layer by increasing the airflow of a mono-silane gas when thegate insulating is being deposited.

[0004] 2. Discussion of the Related Art

[0005] Generally, liquid crystal display devices are widely used becausethey are compact and thin size as well as light weight. Typically,liquid crystal display devices have a great contrast ratio, and aresuitable for both gray scale and moving picture display. In addition,liquid crystal displays consume less power than alternate display,notably CRTs(cathode ray tube).

[0006] Such a liquid crystal display device includes a thin filmtransistor substrate having thin film transistors and pixel electrodesin pixel areas defined by gate and data lines, a color filter substratehaving a color filter layer and a common electrode, and a liquid crystallayer inserted between the thin film transistor and color filtersubstrates.

[0007] The typical liquid crystal display generally includes twosubstrates having electric field generating electrodes formed thereonrespectively to confront each other and liquid crystals injected betweenthe two confronting substrates. If a voltage is applied to theelectrodes to generate an electric field, liquid crystals molecules aredriven to display an image in accordance with light transmittance variedby the electric field.

[0008] There are various types of the liquid crystal displays. Recentlyan active matrix liquid crystal display(AM-LCD) on which thin filmtransistors and pixel electrodes connected to each other are arranged ina matrix have been receiving heightened attention as they provideexcellent resolution and implementation of moving pictures.

[0009] Such a liquid crystal display contains pixel and commonelectrodes that are formed on lower and upper substrates, respectively,and drives the liquid crystal molecules by applying an electric fieldbetween the substrates in a direction perpendicular to the substrates.

[0010] A liquid crystal display according to a related art is explainedby referring to the attached drawings as follows.

[0011] Referring to FIG. 1, a plurality of gate lines 11 are |formed inone direction on a lower array substrate 10 of a liquid crystal displayand a gate electrode 12 protrudes from one side of each of the gatelines 11.

[0012] A plurality of data lines 14 are formed perpendicular to the gatelines 11, and cross with the gate lines 11 to define pixel areas,respectively.

[0013] A source electrode15 protrudes from one side of each of the datalines 14, and a drain electrode 16 is separated from the sourceelectrode 15 to leave a predetermined interval.

[0014] Moreover, the source, drain, and gate electrodes 15, 16, and 12form a thin film transistor T including an active layer of amorphoussilicon13 over the gate electrode 12.

[0015] The source and drain electrodes 15 and 16 overlap both uppersides of the gate electrode 12.

[0016] A pixel electrode 18 made of a transparent conductive material isformed on each of the pixel areas to overlap the drain electrode 16 inpart, and a contact hole 17 is formed at the portion where the pixel anddrain electrodes 18 and 16 overlap with each other.

[0017] Meanwhile, a storage capacitor Cst is formed to maintain a cellvoltage.

[0018] In this case, an upper electrode of the storage capacitor Cst isformed of an opaque metal layer 14 a having a predetermined pattern anda lower electrode of the storage capacitor Cst is formed of the gateline 11 at the front end.

[0019] The opaque metal layer 14 a is formed to overlap the gate line 11at the front end in part when the data line 14 is formed, and partiallyoverlaps the pixel electrode 18.

[0020] In addition, a contact hole 17 a exposing a predetermined portionof the opaque metal layer 14 a is formed together with the previouscontact hole 17. Hence, the gate line 11, opaque metal layer 14 a, andan insulating layer 22(shown in FIG. 2), which is inserted between thegate line 11 and opaque metal layer 14 a, form the storage capacitor Cstwhen a voltage is applied to the pixel electrode 18.

[0021] A storage-on-gate system is shown in the drawing, and a lowerelectrode of the storage capacitor is integral with the gate line at thefront end.

[0022] A cross-sectional view of such an array substrate is shown inFIG. 2 illustrating a cross-sectional view along a cutting line V-V′ inFIG. 1, in which a storage electrode part A and a thin film transistorpart B are separated from each other for explanation. And, the sameelements are indicated by the same numerals.

[0023] Referring to FIG. 2, a gate line 11 is formed in the storageelectrode part A on a lower array substrate 10 and a gate electrode 12extending from the gate line 11 is formed in the thin film transistorpart B.

[0024] A gate insulating layer 22 is formed on an entire surface of thestorage electrode and thin film transistor parts A and B.

[0025] And, an active layer 13 is formed in a thin film transistorforming area on the gate insulating layer 22 of the thin film transistorpart B.

[0026] The active layer 13 includes an amorphous silicon layer 13 a anda doped semiconductor layer 13 b on the amorphous silicon layer 13 a forohmic contact and etch prevention.

[0027] Source and drain electrodes 15 and 16 are arranged to overlapboth side ends of the doped semiconductor layer 13 b, respectively.

[0028] In this case, the source electrode 15 is an electrode extendingfrom the data line 14, and the drain electrode 16 is isolated from thesource electrode 15.

[0029] Besides, an opaque metal layer 14 a that overlap the gate line11is formed in the storage electrode part A simultaneously when the sourceand drain electrodes 15 and 16 are formed.

[0030] A passivation layer 24 is formed on an entire surface of thesubstrate 10 having the opaque metal layer 14 a and source/drainelectrodes 15/16 formed thereon.

[0031] Contact holes 17 and 17 a exposing predetermined portions of thedrain electrode 16 and opaque metal layer 14 a respectively are formedin the passivation layer 24. And, a pixel electrode 18 made of atransparent material is formed on the passivation layer 24 in the pixelarea to contact the drain electrode 16 and opaque metal layer 14 a.

[0032] In the above-constituted liquid crystal display device, the gateinsulating and passivation layers 22 and 24 have great influence on thestorage capacitance Cst between the gate line 11 and pixel electrode 18.

[0033] The storage capacitance Cst plays a role in uniformly maintainingthe voltage applied to the pixel electrode 18, and the gate insulatinglayer 22 is the most important factor that affects electriccharacteristics of the thin film transistor.

[0034] Moreover, the gate insulating layer 22 demands a high insulatingcharacteristic between the gate electrode 12 and active layer 13 whilethe voltage is not applied thereto, and uses a thin insulating layermaterial having stable characteristics as well as a good breakdownvoltage.

[0035] Typically, silicon oxide (SiO₂), silicon nitride (SiN_(x)) or thelike is widely used to form the gate insulating layer 22.

[0036] However, silicon oxide has a slow deposition rate when the layeris formed for fabricating a thin film transistor(TFT) and has a lowbreakdown voltage for insulation. Hence, the silicon nitride layer iscommonly used as the material of the insulating layer.

[0037] A method of forming a silicon nitride layer according to arelated art is explained as follows.

[0038] First of all, the silicon nitride layer is formed by plasmaenhanced chemical vapor deposition (PECVD) using a gas mixture ofmono-silane (SiH₄) and ammonia (NH₃).

[0039] For instance, the silicon nitride layer is formed by mixing 320sccm(standard cubic cm/min) of a mono-silane(SiH₄) gas and 1,200 sccm ofan ammonia(NH₃) gas with each other at a mixing ratio of about 0.27:1.

[0040] The silicon nitride layer prepared using the mixed raw materialgas with the mixing ratio can be used as the gate insulating layer aswell as the passivation layer.

[0041] In order to form a silicon nitride layer fitting thecharacteristics of the gate insulating and passivation layers, a morereasonable process of fabricating the silicon nitride layer is demanded.

[0042] A process of depositing a silicon nitride layer according to arelated art is explained in detail as follows.

[0043]FIG. 3 illustrates a diagram of a deposited thickness of a siliconnitride layer according to a related art.

[0044] Referring to FIG. 3, a contour line is used in the drawing sothat a horizontal direction indicates a horizontal length of asubstrate, a vertical direction indicates a vertical length of thesubstrate, and a height direction indicates a thickness of a siliconnitride layer.

[0045] In the drawing, the silicon nitride layer according to therelated art is deposited thick in the central portion of the substrateand tends to become thinner toward the edges of the substrate.

[0046] Namely, in a single substrate including several liquid crystalcells A, B. C, D, E, and F, as shown in FIG. 4, the silicon nitridelayer is deposited thin on peripheries of the liquid crystal cells C, D,E, and F corresponding to both sides of the substrate.

[0047]FIG. 5 illustrates a graph of a cross-section along a cutting lineI˜I′ in FIG. 3.|

[0048] Referring to FIG. 5, a horizontal axis is a horizontal length ofa substrate and a vertical axis indicates a measured value of a siliconnitride layer thickness 25.

[0049] The silicon nitride layer thickness is measured in units of “Å”.As mentioned in the foregoing explanation, the silicon nitride layer isdeposited thick in the central portion of the substrate and becomesthinner toward the edges of the substrate. Hence, a difference inthickness between the central portion and edge of the substrate is about500 Å.

[0050] However, the process of depositing the silicon nitride layeraccording to the related art has the following problems ordisadvantages.

[0051] First of all, when the thickness of the silicon nitride layerdeposited in the central portion fails to be uniform with that in theedge portion, the gate insulating layer inserted between the gate lineand pixel electrode brings about a storage capacitance fluctuation.

[0052] Such a storage capacitance fluctuation makes each area differ incapability of maintaining a voltage, whereby an image displayed on ascreen fails to disappear the moment power turns off. Specifically, theimage on the edge of the substrate in which the silicon nitride layer isthin turns off slowly due to the increased capacitance.

[0053] Secondly, the thickness difference in the gate insulating layerbrings about a capacitance difference of the thin film transistor.

[0054] Such a capacitance difference is mainly affected by theimpurities in the gate insulating layer but is an important variable fordetermining the value of threshold voltage when a channel is formed inthe semiconductor layer.

[0055] Hence, the deviation according to the thickness of the siliconnitride layer may have influence on the value of threshold voltage.

[0056] Finally, the capacitance difference caused by the thicknessdifference of the gate insulating layer triggers a difference ofparasitic capacitance due to the thickness of the gate insulating layer,whereby a data voltage applied to the pixel electrode varies to make theimage flicker.

SUMMARY OF THE INVENTION

[0057] Accordingly, the present invention is directed to a method offabricating a liquid crystal display device that substantially obviatesone or more problems due to limitations and disadvantages of the relatedart.

[0058] An object of the present invention is to provide a method offabricating a liquid crystal display device enabling formation of auniform gate insulating layer by adjusting a gas mixture ofmono-silane(SiH₄) and ammonia(NH₃) to 0.3˜0.5:1 when a silicon nitridelayer is deposited.

[0059] Additional advantages, objects, and features of the inventionwill be set forth in part in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

[0060] To achieve these objects and other advantages and in accordancewith the purpose of the invention, as embodied and broadly describedherein, a method of fabricating a liquid crystal display deviceaccording to the present invention includes the steps of forming a gateline, a gate electrode, and a storage line on a substrate, and forming agate insulating layer on the substrate including the gate line and thegate electrode using first and second gases having a gas mixture ratioof 0.3˜0.5:1.

[0061] Preferably, the gate insulating layer is formed by plasmaenhanced chemical vapor deposition.

[0062] Preferably, the first and second gases are mono-silane(SiH₄) andammonia(NH₃), respectively.

[0063] Preferably, the gate insulating layer is a silicon nitride layer.

[0064] More preferably, the mono-silane(SiH₄) and ammonia(NH₃) have400˜600 sccm and 1,200 sccm, respectively.

[0065] Preferably, the present invention further includes the steps offorming a semiconductor layer, a source electrode, and a drain electrodeon the gate insulating layer, forming a passivation layer on the sourceand drain electrodes, forming a pixel electrode on the passivation layerto be connected to the drain electrode, and forming an alignment layeron the pixel electrode.

[0066] Thus, the present invention is characterized in that thespecification for depositing the silicon nitride layer is adjusted toattain the gate insulating layer deposited on an entire surface of thesubstrate uniformly.

[0067] It is to be understood that both the foregoing generaldescription and the following detailed description of the presentinvention are exemplary and explanatory and are intended to providefurther explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiment(s) of theinvention and together with the description serve to explain theprinciple of the invention. In the drawings:

[0069]FIG. 1 illustrates a layout of a liquid crystal display accordingto a related art;

[0070]FIG. 2 illustrates a cross-sectional view of a liquid crystaldisplay bisected along a cutting line V-V′ in FIG. 1;

[0071]FIG. 3 illustrates a diagram of a deposited thickness of a siliconnitride layer according to a related art;

[0072]FIG. 4 illustrates a layout of liquid crystal cells schematically;

[0073]FIG. 5 illustrates a graph of a cross-section along a cutting lineI˜I′ in FIG. 3;|

[0074]FIG. 6 illustrates a diagram of a deposited thickness of a siliconnitride layer according to the present invention;

[0075]FIG. 7 illustrates a graph of a cross-section along a cutting lineII˜II′ in FIG. 6; and|

[0076]FIG. 8 illustrates a comparison graph between FIG. 5 and FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

[0077] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0078] A method of fabricating a liquid crystal display device accordingto a preferred embodiment of the present invention is explained asfollows.

[0079] First of all, a conductive metal is deposited on a substrate, andthen a gate line, a gate electrode, and a storage line are formed on thesubstrate by photolithography.

[0080] In this case, the gate line formed to be arranged in onedirection, the gate electrode is formed to protrude from one side of thegate line, and the storage line is built in one body with the gate linein the front end.

[0081] Subsequently, a gate insulating layer is formed on the substrateincluding the gate line and the gate electrode by plasma enhancechemical vapor deposition using a mono-silane(SiH₄) gas having a highflow.

[0082] Thereafter, a semiconductor layer is formed to have an islandshape on the gate insulating layer confronting the gate electrode.

[0083] A data line is formed to cross with the gate line to define apixel area, and simultaneously, source and drain electrodes are formedon the gate insulating layer to be overlapped with both sides of thesemiconductor layer.

[0084] Next, a passivation layer is formed on an entire surface of thesubstrate including the source and drain electrodes and contact holesformed appropriately.

[0085] Subsequently, a pixel electrode is formed on the passivationlayer to be connected to the drain electrode.

[0086] Finally, a first alignment layer is formed on an entire surfaceof the substrate having the pixel electrode formed thereon.

[0087] In forming the gate insulating layer, a gas mixture ratio betweenfirst and second gases is preferably 0.3˜0.5:1 in the plasma enhancedchemical vapor deposition.

[0088] In this case, the first and second gases are mono-silane(SiH₄)and ammonia(NH₃), respectively.

[0089] For instance, the silicon nitride layer is deposited by mixingthe mono-silane(SiH₄) and ammonia(NH₃) to have 400˜600 sccm and 1,200sccm, respectively.

[0090] Preferably, the flow of the mono-silane(SiH₄) gas is 450 sccm.

[0091] The deposited silicon nitride layer according to the presentinvention is explained by referring to the attached drawings as follows.

[0092]FIG. 6 illustrates a diagram of a deposited thickness of a siliconnitride layer according to the present invention.

[0093] Referring to FIG. 6, a silicon nitride layer deposited by thespecification for the deposition of the silicon nitride layer accordingto the present invention has a uniform thickness of an entire surface ofa substrate, and has a very small deviation difference between ahorizontal thickness and a vertical thickness. Hence, it can be seenthat the silicon nitride layer is deposited uniformly.

[0094]FIG. 7 illustrates a graph of a cross-section along a cutting lineII˜II′ in FIG. 6.

[0095] Referring to FIG. 7, a measured value 22 of thickness of asilicon nitride layer according to the present invention has a uniformdistribution overall despite small difference.

[0096] The improve thickness deviation of the silicon nitride layeraccording to the present invention is compared to that of the relatedart, which is explained as follows.

[0097]FIG. 8 illustrates a comparison graph between silicon nitridelayers according to the related art and the present invention.

[0098] Referring to FIG. 8, a measured value 22 of a thickness of asilicon nitride layer according to the present invention has a deviationsmaller than that of the related art, and the silicon nitride layer ofthe present invention has a uniform thickness.

[0099] First of all, the silicon nitride layer deposited by the methodaccording to the present invention shows such an effect as the smallthickness deviation at the edge of the substrate.

[0100] Moreover, as the thickness deviation decreases, the deviation ofstorage capacitance is reduced to decrease a discharging timedifference.

[0101] The discharging times are compared to each other using thedeviation in accordance with the thickness, which is explained asfollows.

[0102] Table 1 is a comparison between the related art and the presentinvention. TABLE 1 Classification Related art Present invention Unit≦12% ≦8% Discharging time ≦15 sec ≦5 sec

[0103] In Table 1, a reference of unit is taken by dividing a differencebetween minimum and maximum values in thickness except 100 mm of bothedges of a substrate by an averaged value and multiplying the dividedresult by 50 to represent a percentage.

[0104] Accordingly, the silicon nitride layer, which is deposited by themethod of fabricating the thin film transistor according to the presentinvention, has a reduced thickness deviation, which decreases thedeviation of storage capacitance, thereby enabling a reduction in thedischarging time difference.

[0105] Moreover, the method of fabricating the thin film transistoraccording to the present invention reduces the thickness difference ofthe gate insulating layer to provide a uniform value of thresholdvoltage and decreases the deviation of parasitic capacitance due to thethickness of the gate insulating layer to reduce flicker on the screen.

[0106] Accordingly, the method of fabricating the liquid crystal displaydevice according to the present invention has the following effects oradvantages.

[0107] Firstly, the thickness deviation of the silicon nitride layer isreduced to decrease the deviation of the storage capacitance, wherebythe discharging time difference is reduced.

[0108] Secondly, the present invention reduces the thickness differenceof the gate insulating layer to provide a uniform value of thresholdvoltage and decreases the deviation of parasitic capacitance due to thethickness of the gate insulating layer to reduce flicker on the screen.

[0109] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of fabricating a liquid crystal displaydevice, comprising: forming a gate line, a gate electrode, and a storageline on a substrate; and forming a gate insulating layer on thesubstrate including the gate line and the gate electrode using first andsecond gases having a gas mixture ratio of 0.3˜0.5:1.
 2. The method ofclaim 1, wherein the gate insulating layer is formed by plasma enhancedchemical vapor deposition.
 3. The method of claim 1, wherein the firstand second gases are mono-silane(SiH₄) and ammonia(NH₃), respectively.4. The method of claim 1, wherein the gate insulating layer is a siliconnitride layer.
 5. The method of claim 3, wherein the mono-silane(SiH₄)and ammonia(NH₃) have 400˜600 sccm and 1,200 sccm, respectively.
 6. Themethod of claim 1, further comprising forming a semiconductor layer, asource electrode, and a drain electrode on the gate insulating layer. 7.The method of claim 6, further comprising forming a passivation layer onthe source and drain electrodes.
 8. The method of claim 7, furthercomprising forming a pixel electrode on the passivation layer to beconnected to the drain electrode.
 9. The method of claim 8, furthercomprising forming an alignment layer on the pixel electrode.